Circuit board bonding method, branch circuit and its designing method, waveguide-microstrip transition, and application to HF circuit, antenna and communication system

ABSTRACT

A circuit assembly having a thin and large-area dielectric substrate and an improved grounding condition. To make the assembly, a circuit board comprises the substrate, and a circuit pattern and a metal layer that are formed on respective sides of the substrate. A bath of conductive bonding material (e.g., a low melting point solder) is made inside a tray-like metal chassis of the assembly. The circuit board is floated on the bath and excessive portion of the conductive material is absorbed. A branch circuit for branching a first path into at least two second paths is provided by mainly using impedance transformers but by using fewest possible stubs. Also, the elements are arranged in symmetry around the axis through the first path. This yields a wide operating frequency band. A waveguide-microstrip line transition that is easy to work and low in transition loss is provided by shaping the wider walls of the ridge waveguide so as to spread out toward the end. Since doing this increases the degree of freedom in design parameters, the width and the height of the ridge can be fitted to that of the microstrip line and that of the waveguide. The above elements are applicable to HF circuits, antennas and communication systems.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 09/290,395, filed Apr. 13, 1999, now U.S. Pat. No. 6,335,664.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high frequency (HF) circuit in acommunication device and more specifically to a technique for bonding acircuit board to a metal chassis or case, a waveguide-microstrip linetransition, a branch circuit, and a high frequency circuit incorporatingthese elements.

2. Description of the Prior Art

Recently, as frequency resources in communications technology arerunning dry, frequency bands available for building a new communicationssystem have been and are shifting to higher bands. In this situation,the government and the people are jointly promoting a development tomilliwave and microwave communication systems domestically andinternationally. For example, it has been decided that extremely highfrequency bands ranging from some GHz to hundreds GHz are assigned asavailable frequency bands to various communication systems underdevelopment for wireless LAN (local area network) and ITS (IntelligentTransport System).

Since available frequencies are rising as described above, antennas andHF (high frequency) circuits are desired which satisfactorily work inmilliwave and microwave bands. However, design and manufacturingtechniques that have been believed to be available may not worksatisfactorily with an increase in frequency. For this reason, there isa need for novel design and manufacturing techniques.

FIG. 1 is a diagram showing an arrangement of a prior art array antennaassembly 1. In FIG. 1, the antenna assembly 1 comprises an dielectricsubstrate 10, a circuit pattern 20, a chassis 30 that holds thedielectric substrate 10 and serves as the ground, and awaveguide-microstrip line transition 40. The circuit pattern 20, whichconstitutes an array antenna, includes a T branch circuit 50. A signaltransmitted through a waveguide (not shown) is passed by the transition40 to a microstrip line of the circuit pattern 20, and further passed bythe T branch circuit 50 to the right and the left portions of the arrayantenna.

FIG. 2 is a schematic diagram showing an arrangement of the transition40 of FIG. 1. In FIG. 2, the transition 40 comprises a ridge waveguide42, a ridge 41 formed inside the ridge waveguide 42, and a microstripline 21 which is formed on the dielectric substrate 10 and which isextending to (or a part of) the circuit pattern 20. As described above,the signal transmitted through the not-shown waveguide is converted intoa transmission mode of the microstrip line 21 by the ridge 41 providedinside the waveguide 42 and transmitted to the array antenna 20.

Problems exist in conjunction with working if an antenna with thejust-described arrangements are to be implemented for milliwave ormicrowave. With an increase in frequency, dielectric materials availablefor the dielectric layer 10 is limited to substances lack of amechanical strength, e.g., ceramics, quartz, silicon, etc. Further, ifan antenna that radiates a beam of two degrees in mesial width in a 76GHz band is to be fabricated, the dielectric substrate 10 for theantenna will be approximately 100 to 300 μm thick and 15 cm long in oneside. Bonding such a thin and wide substrate 10 to the chassis 30 oftenresults in a breakage of the dielectric substrate 10. Also, as thefrequency increases, the characteristics of the antenna 20 dependsstrongly on the grounding state of the dielectric substrate 10. For thisreason, a sufficient electrical contact is indispensable for thejunction of the dielectric substrate 10 and the circuit pattern 20.However, this is hard to be achieved by conventional techniques.

Since the degree of freedom is very low in designing awaveguide-microstrip line transition, i.e., the design parameters arelimited only to the width, the length and the height of the ridge 41,this sometimes causes the width of ridge for a milliwave band to beextremely narrow. Accordingly, the height of the ridge 41 of thetransition 40, which is manufactured through machining of a brassmaterial, becomes higher as compared with the ridge 41 width, making thework difficult. The lack of freedom in the design makes transition witha microstrip line having a lower characteristic impedance difficult andcause the problem that too large a difference between the widths of thedesigned ridge 41 and the microstrip line 21 leads to an unexpecteddeterioration in the impedance matching characteristics.

As is not limited to a high frequency (HF) antenna, an array antenna 20as a whole generally exhibits a narrower frequency band characteristicwith an increase in the number of array elements. Taking for example anantenna used in a front monitoring radar being put to practical use in60 GHz, the antenna needs a beam width of about 2 degrees andaccordingly a very large size. If a structure incorporating aconventional branch circuit were used as it is for such antenna, theresultant antenna would exhibit a very narrow frequency bandcharacteristic, causing the band width of the antenna to be narrowerthan that of the radar. This is because conventional branch circuitsmainly use stubs for impedance matching. FIG. 3 is a diagram showing anexemplary pattern of a conventional T branch circuit comprising amatching circuit that uses stubs 51 (the T branch circuit is shown as adark area). Using stubs for impedance matching generally tends to narrowthe frequency characteristics of the circuit. Specifically, the largerthe distances (D1 and D2) between the matching circuit and circuits (22)that need matching, the narrower the frequency band of the wholecircuit. However, if stubs are to close to the antenna (or the circuitsthat need matching) so as to broaden the frequency band of the antenna,the antenna will fail to provide the desired characteristic. Thus,matching by stubs while providing a desired characteristic to theantenna or the circuits having their impedance matched inevitablynarrows the frequency band of the resultant circuit such as an antenna.

SUMMARY OF THE INVENTION

The invention is directed to solving these and other problems anddisadvantages of the prior art.

It is an object of the invention to provide a technique of bonding athin and large-area circuit substrate to a metal layer with a sure anduniform contact but no fear of substrate breakage; awaveguide-microstrip transition that has a high degree of freedom indesign and easy to work; and a branch circuit that permits the frequencyband of circuit to be wide.

It is another object of the invention to provide a high frequencycircuit and an antenna that incorporate an circuit substrate implementedby such a bonding technique, such a waveguide-microstrip transition andsuch a branch circuit, and to provide a communication system using sucha high frequency circuit and such an antenna.

According to an aspect of the invention, a method of bonding a circuitboard with a metal plate is provided. The method includes the steps ofworking the metal plate so as to have a shape that permits a fluid toform a bath in an area including a part where the circuit board is to bebonded; heating the worked metal plate to such a temperature as melt aconductive bonding material; forming a bath of the conductive bondingmaterial in the area of the metal plate; floating the circuit board onthe bath; and absorbing excessive portion of the conductive materialwithout applying a force to the dielectric substrate.

A circuit assembly according to just-described aspect of the inventionis provided with a thin and large-area dielectric substrate with animproved grounding condition. A bonding agent with a low melting point,a low melting point solder, etc. may be used as conductive material.

According to another aspect of the invention, a branch circuit forbranching a first path into at least two second paths in a highfrequency circuit is provided. The impedance matching between the firstpath and each of the branch paths is achieved by mainly using impedancetransformers but by using fewest possible stub(s) in the branch circuit.The first path, the second paths, the impedance transformers, and thefewest possible stub(s) are arranged in symmetry with respect to a planeof symmetry that runs through the first path.

In one embodiment, the impedance transformers are step impedancetransformers.

According to another aspect of the invention, a waveguide-microstripline transition that is easy to work and low in transition loss isprovided. The transition comprises a fanwise tube having a first openingcoupled with a waveguide and a second opening larger in size then thefirst opening, a first and a second wider wall of the tube spreadingfrom the first opening toward the second opening; an end portion of amicrostrip line formed on a dielectric substrate arranged near the firstwider wall, the end portion being situated a little inside the secondopening and on a plane of symmetry for the first and the second widerwalls; and a ridge formed on the second wider wall, the ridge protrudinggradually from a first opening side toward a second opening side tobecome short-circuited, at the end thereof, with the end portion of themicrostrip line, wherein dimensions of the fanwise tube and a shape ofthe first and the second wider walls are determined so as to fit thewidth of the microstrip line to the end portion of the microstrip line.

In one embodiment, at least a part of each longitudinal side of afanning-out portion of the wider walls is linear. However, at least apart of each longitudinal side may accords substantially with anexponential function or a trigonometric function.

BRIEF DESCRIPTION OF THE DRAWING

The features and advantages of the present invention will be apparentfrom the following description of an exemplary embodiment of theinvention and the accompanying drawing, in which:

FIG. 1 is a diagram showing an arrangement of a prior art array antennaassembly 1;

FIG. 2 is a diagram showing an arrangement of the waveguide-microstripline transition 40 of FIG. 1;

FIG. 3 is a diagram showing an exemplary pattern of a conventional Tbranch circuit comprising a matching circuit that uses stubs;

FIG. 4 is a schematic diagram showing an exemplary arrangement of anarray antenna assembly according to an illustrative embodiment of theinvention;

FIG. 5 is a schematic sectional view taken along the plane X-Y of FIG.4;

FIG. 6 is a diagram showing an exemplary process of bonding the metallayer 11 of a circuit board to the chassis 130 in accordance with theprinciples of the invention;

FIG. 7 is a schematic diagram showing an exemplary arrangement of thewaveguide-microstrip line transition 140 of FIG. 4;

FIG. 8 is a schematic diagram showing an exemplary pattern of the Tbranch circuit 150 of FIG. 4;

FIG. 9 is a diagram showing a reflection characteristic curve of theantenna 100 incorporating the inventive T branch circuit 140; and

FIG. 10 is a diagram showing a reflection characteristic curve of theconventional antenna 1 of FIG. 1.

Throughout the drawing, the same elements when shown in more than onefigure are designated by the same reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 is a schematic diagram showing an exemplary arrangement of anarray antenna assembly 100 according to an illustrative embodiment ofthe invention. In FIG. 4, the antenna assembly 100 comprises andielectric substrate 10, a circuit pattern or an array antenna 120formed by patterning a metal film on the dielectric substrate 10 throughphotocopying, etching, etc., a chassis 130 that holds the dielectricsubstrate 10 and serves as the ground, and a waveguide-microstrip linetransition 140 formed in the edge of the chassis 130. The circuitpattern 120 serves as a microstrip antenna array. A signal transmittedthrough a waveguide (not shown) is passed by the transition 140 to amicrostrip line coupled to the circuit pattern 120, and further passedby the T branch circuit 50 to the right and the left portions of thearray antenna 120.

Bonding the Dielectric Substrate

FIG. 5 is a schematic sectional view taken along the plane X-Y of FIG 4.In FIG. 5, the antenna assembly 100 comprises metal layers 120 and 11,the dielectric layer 10, a solder layer 111 and the chassis 130. Themetal layer 120 on the dielectric layer 10 comprises a circuit patternforming a microstrip structure in cooperation wit the metal layer 11.the metal layer 120, the dielectric layer 10 and the metal layer 11constitute a circuit board 120+10+11. According to the invention, themetal layer 11 of the circuit board 120+10+11 is bonded with the chassis130 by means of a bonding material of a good conductivity, whichrealizes sufficient grounding and strong support of the circuit board.In this specific embodiment, a lower-melting point solder is used as thebonding material. It should be noted that the chassis 130 is not a flatplate but has a concavity on one side thereof as shown in FIG. 5.

FIG. 6 shows how the metal layer 11 is bonded with the chassis 130 inaccordance with the principles of the invention. The processing stepsare as follows:

(1) Put solder 111 a in the concavity of the heated chassis 130 to makea solder bath 111 b. The solder 111 a is preferably a low melting pointsolder with a low melting point. Because if the difference between therates of expansion for the substrate 10 and the chassis 130 is large,heating too much for a higher melting point solder may cause thesubstrate 10 to break during a cooling process.

(2) Float the circuit board 120+10+11 on the solder bath 111 b. Thecircuit board floats of itself due to the surface tension of the solder111 b.

(3) Get the metal layer 11 of the circuit board attached to the solder111 b, getting bubbles out from between the metal layer 11 and thechassis 130.

(4) Remove excessive solder by absorbing solder 111 b little by littlewithout applying external force directly to the circuit board. Theabsorption of solder 111 b is achieved by using a dedicated copperfiber.

(5) Cool taking a sufficient time such that any undesirable stress willnot remain in the resultant assembly.

The just-described process enables even a thin and large-area dielectricsubstrate 10 (i.e., circuit board 120+10+11) from which alone asufficient mechanical strength can not be expected to be bonded with thechassis 130 without a fear of breaking the substrate 10 (or the circuitboard), realizing good grounding.

Though the inventive bonding technique has been described in conjunctionwith an antenna assembly 100 using an array antenna pattern 20, theinventive technique may be applied to any high frequency (HF) circuitassembly with any circuit pattern.

It is noted that any suitable conductive bonding agents may be usedinstead of the solder 111 a. In this case, the bonding agents arepreferably low in the temperature and small in the ratio of volumechange at about the hardening temperature.

The inventive bonding technique has been described in conjunction withbonding a circuit board with a chassis. However, the inventive techniqueis applicable to bonding a thin and large-area circuit board with ametal surface in a concavity or a metal surface fringed with walls.

Waveguide-Microstrip Line Transition

FIG. 7 shows an exemplary arrangement of the waveguide-microstrip linetransition 140 of FIG. 4. In FIG. 7, the transition 140 comprises anextension 42 of a waveguide (not shown), a fanwise tube (i.e., a tubespreading out toward the end) 142, a ridge 144 so formed as to protrudeinside the fanwise tube 142, and a microstrip line 21 formed on thedielectric substrate 10 and extending to (or forming a part of) thecircuit pattern 120. The fanwise tube 142 has a horn-like structure witha rectangular cross section perpendicular to the current direction. Thetwo opening of the fanwise tube 142 differ in dimensions from eachother. The transition 140 serves as a transition between the waveguideextension 42 and the microstrip line 121. The transition 140 is aso-called ridge waveguide converter provided, in the center of thefanwise tube 142, with a ridge 144 of a wedge shape. Specifically, theelectromagnetic fields distributed all over the opening of the waveguideextension 42 is gradually converged on the head of the ridge 144 andfinally made resemble the electromagnetic transmission mode of themicrostrip line 121 thereby to be power-transmitted to the microstripline 121.

According to the present invention, the line conversion characteristicsof the transition 140 are a function of (i.e., can be controlled by) notonly the dimensions of the ridge 144 but also the shape of fanning ofthe fanwise tube 142. That is, using a tube spreading out like anunfolded fan has increased the degree of freedom in designing the ridge144. Since the designer can freely select the width and the height ofthe ridge 144, the transition 140 can be so designed as to have desiredline conversion characteristics with a reduced parasitic impedance nearthe interface between the ridge 144 head and the microstrip line 121. Inthis way, the inventive transition 140 is easy to work, relatively lowerin conversion loss and better in conversion characteristics.

Also, since the height of the ridge 144 can be set identical to that ofthe fanwise tube 142, the fanwise tube 142 with a high precision in thedimensions is provided, which contributes to better line conversioncharacteristics of the resultant transition 140.

The fanwise tube 142 has been shown as linearly shaped in FIG. 7.However, the tube 142 may have any nonlinear shape that spreads outtoward the end. The side of the tube 142 may be curved according to,e.g., an exponential function, a trigonometric function, etc. The twoopening of the tube 142 may have any suitable geometric shape differentfrom each other to exhibit different characteristic impedance.

Though the transition 140 has been described as a waveguide-microstriptransition provided in a path to an antenna, the transition 140according to the invention can be applied to an ordinary high frequencycircuit.

Branch Circuit

FIG. 8 is a schematic diagram showing an exemplary pattern of the Tbranch circuit 150 of FIG. 4 as a dark area. In FIG. 8, the T branchcircuit 150 comprises a root path 151, two branch paths 152, a stub 153and step impedance transformers 154 and 155. The amplitude and the phaseof the signals supplied to the antenna elements 122 are controlled bydesign parameters of the step impedance transformers 154 and 155. The Tbranch circuit 150 is configured symmetrical about the plane of symmetrythat runs through the root path 151.

In this specific embodiment, the T branch circuit 150 is so configuredthat each of the root path 151 and the branch paths 152 has acharacteristic impedance of 50 ω. The signal on the root path 151 isdistributed into the two paths 152 with an equal power and a same phase.The branch paths 152 are coupled in serial with the antenna elements122. Arrangements are made such that the overall impedance of the branchcircuit has an impedance smaller than 50 ω.

It should be noted that the T branch circuit 150 achieves impedancematching by mainly using step impedance transformers (154 and 155 x 2)but using the fewest possible stubs (153). Doing this makes the bandwidth of the whole circuit or array antenna 120 wider as compared withan ordinary circuit that uses stubs for impedance matching. Generallyspeaking, an antenna has to have a frequency band width wider than thesum of the band width occupied by the communication system where theantenna is incorporated and a frequency error involved in themanufacturing process of the antenna. The inventive T branch circuit 150enables implementation of such a wide band circuit or antenna.

An illustrative embodiment has been described in conjunction with the Tbranch circuit 150 having two branch paths. However, the invention isapplicable to any T branch circuits having more than two branch paths. Awider band HF circuit and a communication system can be implemented byusing a T branch circuit according to the principles of the invention.

FIG. 9 is a diagram showing a reflection characteristic curve of theantenna 100 incorporating the inventive T branch circuit 140. FIG. 10 isa diagram showing a reflection characteristic curve of the conventionalantenna i1 of FIG. 1. In FIGS. 9 and 10, the axis of abscissas indicatesthe frequency from 35 through 45 GHz, and the axis of ordinatesindicates the return loss viewed from the entrance to the root path. TheT branch circuit 150 according to the invention have achieved matchingsuch that the return loss is less than −10 dB for 4 GHz from 37.3through 42 GHz as shown in FIG. 9. On the other hand, the conventional Tbranch circuit 50 of FIG. 3 have achieved matching of less than −10 dBfor 2 GHz from 39 through 41 GHz as shown in FIG. 10. Thus, a T branchcircuit according to the present invention yields substantially twice ofthe band width of conventional one.

Many widely different embodiments of the present invention may beconstructed without departing from the spirit and scope of the presentinvention. It should be understood that the present invention is notlimited to the specific embodiments described in the specification,except as defined in the appended claims.

What is claimed is:
 1. A method of bonding a metal layer of a boardcomprising at least a dielectric layer and the metal layer having ametal plate, the method including the steps of: working said metal plateso as to have a shape that permits a fluid to form a bath in an area,including a part where said board is to be bonded; heating said workedmetal plate to such a temperature as to melt a conductive bondingmaterial; forming a bath of said conductive bonding material in saidarea of said metal plate; floating said board on said bath; andabsorbing an excessive portion of said conductive bonding materialwithout applying a force to said board.
 2. A method as defined in claim1, further including the step of selecting a bonding agent with a lowmelting point as said conductive bonding material.
 3. A method asdefined in claim 1, further including the step of selecting a lowmelting point solder as said conductive bonding material.
 4. A method ofmanufacturing a circuit assembly, including the step of: preparing acircuit board comprising a dielectric substrate, a circuit patternformed on one side of said substrate and a metal layer formed on theother side of said substrate; preparing a metal chassis having a shapelike a tray capable of containing said circuit board; heating said metalchassis to such a temperature as to melt a conductive bonding material;putting said conductive material in said chassis to form a bath of saidconductive bonding material; floating said circuit board on said bath;and absorbing an excessive portion of said conductive bonding materialwithout applying a force to said circuit board.
 5. A method as definedin claim 4, further including the steps of attaching said dielectricsubstrate to said bath, and forcing bubbles out from between saiddielectric substrate and said metal plate.
 6. A method as defined inclaim 4, further including the step of selecting a bonding agent with alow melting point as said conductive bonding material.
 7. A method asdefined in claim 4, further including the step of selecting a lowmelting point solder as said conductive bonding material.